Lyα Emission in Starbursts: Implications for Galaxies at High Redshift

We present the results of a high-resolution UV two-dimensional spectroscopic survey of star-forming galaxies observed with Hubble Space Telescope Space Telescope Imaging Spectrograph. Our main aim is to map the Lyα profiles to learn about the gas kinematics and its relation with the escape of Lyα photons and to detect extended Lyα emission due to scattering in gaseous halos. We have combined our data with previously obtained UV spectroscopy on three other star-forming galaxies. We find that the P Cygni profile is spatially extended, smooth, and spans several kiloparsecs covering a region much larger than the starburst itself. We propose a scenario whereby an expanding supershell is generated by the interaction of the combined stellar winds and supernova ejecta from the young starbursts, with an extended low-density halo. The variety of observed Lyα profiles both in our sample and in high-redshift starbursts is explained as phases in the time evolution of the supershell expanding into the disk and halo of the host galaxy. The observed shapes, widths, and velocities are in excellent agreement with the supershell scenario predictions and represent a time sequence. We confirm that among the many intrinsic parameters of a star-forming region that can affect the properties of the observed Lyα profiles, velocity and density distributions of neutral gas along the line of sight are by far the dominant ones, while the amount of dust will determine the intensity of the emission line, if any.